ACCURACY
is a measure of how close the output is to where it should be. It is usually expressed in units of distance, such as ±30 arcseconds or ±0.0001 inch. If it's expressed as a percent, make sure to state whether it's a percent of full scale (not usually meaningful with a rotary encoder) or a percent of nominal resolution. 

ANGULAR MISALIGNMENT
is the maximum deviation in perpendicularity between the encoder shaft and the face of the mounting surface. It is the total of shaft misalignment, shaft runout and mounting face runout. 

BIT
is an abbreviation for Binary digit; it refers to the smallest element of RESOLUTION. (See QUANTUM) 

CPR
an mean either cycles/rev or counts/rev. To avoid confusion, this term should not be used. (see Understanding Quadrature) 

ERROR
is the algebraic difference between the indicated value and the true value of the input. 

FREQUENCY RESPONSE
is the encoder's electronic speed limit, expressed in kilohertz (1 kHz = 1000 Hz = 1000 cycles/sec). For calculations, rotational speed must be in rev/sec (rps = rpm/60); linear speed must be either in/sec or mm/sec, depending on the scale line count.

(cycles/rev) x (rev/sec)/1000 = kHz

INDEX SIGNAL
is a once-per-rev output used to establish a reference or return to a known starting position; also called reference, marker, home, or Z 

INTERPOLATION
involves an electronic technique for increasing the resolution from the number of optical cycles on the disc or scale to a higher number of quadrature square waves per revolution or per unit length. These square waves can then be QUADRATURE DECODED. 

MEASURING STEP
is the smallest RESOLUTION element; it assumes QUADRATURE DECODE. (see also QUANTUM)

mks - also referred to as µs

PPR = Pulses per revolution.

Commonly (but mistakenly) used instead of cycles/rev when referring to QUADRATURE square wave output. (See the technical article Understanding Quadrature) 

QUADRATURE
refers to the 90-electrical-degree phase relationship between the A and B channels of incremental encoder output. (See the technical article Understanding Quadrature) 

QUADRATURE DECODE
(or 4X Decode) refers to the common practice of counting all 4 quadrature states (or square wave transitions) per cycle of quadrature square waves. Thus, an encoder with 1000 cycles/rev, for example, has a resolution of 4000 counts/rev. (See the technical article Understanding Quadrature) 

QUANTIZATION ERROR
is inherent in all digital systems; it reflects the fact that you have no knowledge of how close you are to a transition. It is commonly accepted as being equal to ±1/2 bit. 

QUANTUM
(plural is QUANTA) = BIT. It is the smallest RESOLUTION element. (QUANTA and BIT are more commonly used with absolute encoders; counts/rev or MEASURING STEPS are more common with incremental encoders.) 

REPEATABILITY
is a measure of how close the output is this time to where it was last time, for input motion in the same direction. It's not usually specified explicitly, but it is included in the accuracy figure. (As a rule of thumb, the repeatability is generally around 1/10 the accuracy.) 

RESOLUTION
is the smallest movement detectable by the encoder. It can be expressed in either electrical terms per distance (e.g., 3600 counts/rev or 100 pulses/mm) or in units of distance (e.g., 0.1° or 0.01 mm). 

SLEW SPEED
is the maximum allowable speed from mechanical considerations. It is independent of the maximum speed dictated by FREQUENCY RESPONSE.